As oil well drilling becomes increasingly complex, the importance of collecting and analyzing downhole data about the formation increases. Well logging instruments are often used to probe subsurface formations to determine formation characteristics.
The basic techniques for electromagnetic logging for earth formations are well known. For instance, induction logging to determine resistivity (or its inverse, conductivity) of earth formations adjacent a borehole has long been a standard and important technique in the search for and recovery of hydrocarbons. Generally, a transmitter transmits an electromagnetic signal that passes through the borehole and the formation materials around the borehole and induces a signal in one or more receivers. The properties of the signal received, such as its amplitude and/or phase, are influenced by the formation resistivity, enabling resistivity measurements to be made. The measured signal characteristics and/or formation properties calculated therefrom may be recorded as a function of the tool's depth or position in the borehole, yielding a formation log that can be used to analyze the formation.
Sonic tools are an example of well logging tools that may be used to provide information regarding subsurface acoustic properties that can be used to analyze the formation. This information may include the compressional wave speed, shear wave speed, borehole modes, and formation slowness. The information obtained by acoustic measurements has a number of applications, including, but not limited to, seismic correlation, petrophysics, rock mechanics and other areas.
During a typical sonic logging of a formation, an acoustic logging instrument or tool is lowered into a borehole that transverses the formation of interest. The acoustic logging tool may be mounted to the drill collar or other devices and directed downhole. Conventional acoustic logging tools include acoustic transducer elements such as a piezoelectric element. Generally, the acoustic transducer can convert electric energy to acoustic energy as well as acoustic energy to electric energy and may act as an acoustic source or an acoustic sensor. The acoustic logging tool typically includes a transmitter which performs as an acoustic source and emits acoustic energy into the formation and one or more receivers or acoustic sensors that receive acoustic energy. Once the acoustic logging tool is lowered into the formation, the transmitter may be periodically actuated to emit pulses of acoustic energy into the borehole. The emitted acoustic waves propagate through the borehole wall producing a reflection that is then detected by the receiver(s) which may produce an electric signal in response. Specifically, the pressure waves generated by the transmitted may be recorded at the receiver(s). Attributes of the acoustic energy that is detected at the receiver(s) may then be used to characterize subsurface properties of the formation of interest such as compressional slowness and shear slowness.
However, due to the presence of the borehole, the formation properties such as compressional slowness and shear slowness can only be measured indirectly, by relating them to the measured characteristics of the borehole modes. Accordingly, acoustic logging tools are typically designed to excite borehole modes in a way that optimizes recovery of formation parameters. Specifically, the acoustic tools may use signals covering a narrow range of frequencies (“narrow band signals”) or signals covering a wide range of frequencies (“broadband signals”). Most commercial borehole acoustic tools utilize broadband excitation functions that can optimally excite a wide range of different formation properties. In borehole acoustic sensing, broadband sources can provide information about multiple frequencies at the same time. This is usually important since the optimal frequency of excitation for a particular formation is typically unknown. Processing of broadband data, however, has many challenges. For instance, time semblance may suffer from interference of different frequencies and frequency semblance may be computationally expensive and sensitive to noise and other effects.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.